The invention relates to a reversing valve for a compressed air membrane pump in which the membranes are pressurised on alternate sides by means of a main control system which oscillates essentially between two positions and is controlled by a pilot system.
Control means for compressed air membrane pumps generally comprise a main control system and a pilot control system. The main control system effects the alternate pressurisation of the membranes, while the pilot control system effects the reversal of the main control system in the end positions of the pump. Known pilot control systems include mechanical systems such as that described, for example, in European patent application 0 061 706 and magnetic systems such as that described in German Offenlegungsschrift 41 06 180.
Pumps with these kinds of control systems do not meet the high demands imposed for the transportation of very high purity acids, alkalis and solvents in the semiconductor industry, since pumps for this purpose require complete freedom from metal, which cannot be achieved with mechanically or magnetically controlled pumps. The high demands made on pumps used in the semiconductor industry arise from the fact that contamination of the pumping media by metal ions must be excluded at all costs, since, for example in the manufacture of a computer chip, even a few metal ions can be enough to ruin a whole batch. It follows that not only must those components of the pump which are in contact with the pumping media be metal-free, but also the components of the pump which do not come into contact with the media: nowadays a service life of 80 to 100 million cycles is expected for membrane pumps, and in operating the pump the possibility of a membrane rupture in which the pumping media could penetrate into the interior of the pump must be taken into account.
Hence for the fields of application described above the only membrane pumps which can be used are those of which the control systems operate pneumatically, since only such pumps can be made metal-free.
A pump with a control system of this kind is described in German patent specification 33 10 131. The reversing system described there comprises a pneumatically driven main valve control piston having a pilot piston system arranged inside the piston. Whenever the membrane pump reaches its end position, the pilot piston system is reversed by longitudinal displacement of the pilot piston by means of stop pins.
A general problem with known pneumatically operated control systems is that operation of the pump at low speed can lead to its coming to a standstill due to the reversing system assuming a neutral position. If the piston of the pilot system, driven by the movement of the membrane, reaches its neutral position, the pressurisation of the piston of the main system is interrupted. This is then no longer fixed in its end position. Since in order to supply driving air to the membranes the piston of the main system must be located in its end position, the pump comes to a standstill if, due to the neutral position of the pilot piston, the main system piston is no longer unilaterally pressurised and then, diverting the pressure from the main duct, likewise assumes a neutral position.
The reversing system known from German patent 33 10 131 accepts considerable technical complications in order to avoid the danger of a pump stoppage due to a neutral position of the control system. Nevertheless even with this system a pump stoppage cannot be ruled out in certain circumstances.
Thus if, for example, there is a sudden drop in pressure of the driving air the main piston can come to a standstill in its middle position and block the compressed air supply of the two membranes. Since the pilot piston is only moved during the last part of the path of the membranes or of the membrane piston, for the remaining time it is freely movable and can swing back from its end position and likewise assume a neutral position. In this state the pump is then completely blocked, since the neutral position of the main piston prevents any movement of the pump, while the neutral position of the pilot piston prevents the main piston from moving out of its neutral position. The tendency of the pilot piston to assume and remain in a neutral position is increased by the presence between the pilot system and the main control piston of a rigid sleeve which does not take part in the respective displacements of the pistons.
The object of the invention is therefore to provide a reversing unit for pneumatically driven compressed air membrane pumps in which an equilibrium state of the main piston in the neutral position, leading to stoppage of the pump, is avoided.
The achievement of this object is based on the idea of constructing the reversing unit in such a way that an equilibrium position of the main piston or of the pilot piston in the neutral position is ruled out by having the pilot system and the main control system cooperate directly, i.e. so that a change in the position of the one piston directly changes the relative position of the other piston, so that the physical relationship of the two pistons is never adjusted independently of one another. The significance of this is that as a result a piston located in the neutral position will already be brought out of its neutral position again by the fact that the second piston is seeking to assume the neutral position.
Accordingly the invention provides a pneumatic reversing system for a pressure-driven membrane pump which pressurises the membranes on alternate sides with compressed air by means of a main control system which swings back and forth essentially between two positions and is controlled by a pilot system in which the pilot control system and the main control piston are disposed directly adjacent to one another.
In an embodiment of such a pneumatic reversing system the adjacent pilot control unit and the main control piston are each provided with ducts such that in each end position of the pilot control piston the main control piston is unilaterally pressurised with air in such a way that the main control piston is displaced into its respective opposite end position.
The object of the invention is also achieved if the pilot control function is taken over by a piston which is coupled with the membrane piston at least over the predominant part of the displacement path of the membrane piston.
This is preferably done by forming the membrane piston in such a way that in the course of its transverse displacement it effects a change in the compressed air pressurisation of the main system piston and thereby its abrupt displacement from one end position to the other.
As a result of the function of the pilot system being taken over by the membrane piston, i.e. by avoiding a separate pilot system which is only actuated during the last section of the path of the membrane and/or of the pilot system immediately adjoining the main control piston, an uncontrolled neutral positioning of the pilot system such as led in the prior art to stoppage of the pump, for example due to swinging back of the pilot piston or to a temporary drop in pressure, is ruled out. If one of the pistons is in its neutral position, then as soon as the second piston moves into its neutral position it is at once subjected to changed pressure relationships such that the first piston moves back again into its end position. In any case the direct coupling of the control pistons leads to the taking up of the neutral position by one piston bringing the other piston out of the neutral position.
The membrane piston in accordance with the invention is preferably formed in three regions with a waist. In a particular position of the membrane piston the waist or narrowed area of the piston, which otherwise fits in its guide in an air-tight manner, provides a connection between the various ducts in the main piston and the pump housing.
Thus in the end position of the membrane piston the compressed air is supplied through a control duct in the main control piston and through the waist in the membrane piston, which in this position is located directly below the duct opening of the main control piston, for example to an end face of the main control piston, so that the latter piston abruptly changes its final position and thus opens a compressed air duct to another membrane. In this position the control duct of the main control piston, which was previously in communication with the waist of the membrane piston, lies on the unnarrowed outer surface of the membrane piston, which prevents the compressed air from being passed on, until through the longitudinal displacement of the membrane piston the waist again forms a connection between the second end face of the main control piston and the pressurised control duct in the main control piston.
Further waists allow the air to flow away from the various chambers which are not pressurised, so that the pressurised chambers can be increased in size without resistance by displacement of the individual components.
As a result of the main system piston and pilot system piston not forming two components which are displaceable independently of the position of the membrane, but rather of a displacement of the main system piston also forcing a change in its position relative to the pilot system or to the membrane piston, the main system piston, when it is on the way to its neutral position in respect of membrane pressurisation, is at once brought back by the change in position relative to the membrane piston or to the pilot system into the stable end position in which the membrane pressurisation is not impaired. Stopping of the pump as a result of a neutral middle position of the main system piston is thus ruled out.
In just the same way swinging of the pilot system back into its neutral position from an end position is also ruled out, since it is necessarily coupled with the position of the membrane. The system in accordance with the invention thus provides a reversing unit which is free from dead points in all operating states.